1. Field of the Invention
The present invention relates to a method of making a gate structure and a related gate structure, and more particularly, to a method of making a gate structure used in CMOS image sensors (CISs) and a related gate structure used in CISs.
2. Description of the Prior Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into electrical signals. The applications of CCDs include monitors, transcription machines, and cameras. Although CCDs have many advantages, CCDs also suffer from high costs and the limitations imposed by their size. To overcome the weaknesses of CCDs and to reduce costs and dimensions, CMOS image sensors (CISs) have been developed. Photodiodes on a CIS have a similar function as CCDs. Since photodiodes of CISs can be produced using conventional techniques, both cost and size of the sensor can be reduced.
Each conventional CIS can be divided into a photosensitive region and a device region. Generally, at least one photodiode is positioned on the photosensitive region. The photodiode and a transfer transistor, a reset transistor, a source follower transistor and a select transistor in the device region form a CIS unit. The photodiode usually receives the incoming light and converts the light into current signals. The devices in the device region are used to transfer the signal to outer electric circuits.
Traditionally, taking a CIS unit as an example, when forming spacers on transistors of a CIS, a spacer material layer is firstly formed on every gate. At this time, the spacer material layer covers every gate and the photodiode. Then, a dry etching process such as plasma treatment is performed to remove the spacer material layer partly, and form spacers on every gate. While the dry etching is being performed, the spacer material layer on the top surface of the gates and the top surface of the photodiode is almost totally removed by the plasma. Therefore, the exposed surface of the photodiode is bombarded by the plasma, and the silicon lattice of the photodiode is damaged. As a result, the dark current of the photodiode will become unstable, and noise will occur during the operation.
In recently developed fabricating processes, in order to avoid the surface of the photodiode being bombarded by the plasma, a patterned photoresistor is formed on the photodiode. However, an extra photo mask and extra steps are needed to form the aforesaid patterned photoresistor. Therefore, the productivity is decreased. Furthermore, after the spacers are formed, the top surface of the photodiode is still covered by the spacer material layer. The remaining spacer material layer on the photodiode will influence currents formed by the photodiode and the sensitivity of the photodiode will be decreased.